The use of metal interconnects in the semiconductor industry is nearly unavoidable in the integrated circuit (IC) manufacturing process. However, the formation of metal interconnects, specifically using copper (Cu), is known to cause contamination problems on the backside of the semiconductor wafers. Backside metal contamination allows for subsequent contamination of other processing chambers in a semiconductor manufacturing facility. This cross-contamination of other chambers results in the contamination of active surfaces of semiconductor wafers in later processing steps. In other words, a single copper contaminated chamber may affect front end and/or backend processing of hundreds of subsequent wafers.
One specific way in which backside metal contamination can contaminate an entire wafer begins by a wafer going through copper deposition and copper planarization processing, such as copper Chemical Mechanical Polish (CMP) processing. Following the CMP process the wafer is scrubbed before progressing to the next processing steps. One example of a subsequent series of steps would be the lithographic processing of the wafer using photolithographic tracks. When a backside contaminated wafer is introduced into the photolithography system, the contamination transfers onto the photolithographic chambers and tracks. This contamination of the photolithography tool allows subsequent wafers that use the tool to be contaminated with copper. This is particularly troublesome when a wafer, that has not completed high temperature front end manufacturing, is exposed to the copper contamination. Copper contamination is readily vaporized at high temperature and will, after vaporization, contaminate active surfaces of wafers in the thermal processing tube as well as the walls of the thermal processing tube.
The migration of contaminants to the device areas causes device failures because of the incorporation of the copper metal into the active areas of semiconductor substrates. The vaporized metal can coat the surfaces of the high temperature furnaces thus also contaminating the subsequent wafers processed through the same chamber. This "snowball" contamination effect needs to be prevented to maintain higher yields through an IC fabrication facility.
FIG. 1 illustrates a tool 10 which is a Total Reflection X-ray Fluorescent system (TXRF) known in the art. The TXRF system 10 is used to measure contamination at specified point on the backside of a wafer 20. The TXRF system comprises an anode X-ray source 12, a monochromator 16, a sample stage 18, a scintillation counter 26, a solid state detector 22, and an amplifier 24.
In operation, the anode X-ray source 12 produces X-rays which are targeted at the monochromator 16. The monochromator is used to filter out all but a specified wavelength of the X-ray signals, and focuses this specified X-ray signal to a specified point on the wafer 20. The reflected X-ray signal is represented by X-ray portion 14b and is reflected off of the monochromator 16 such that reflected X-ray portion 14c has a very shallow angle of incidence relative to the wafer surface 20. Reflected X-rays 14c from the wafer 20 are received at the scintillation counter 26. The scintillation counter 26 is used to detect the actual reflection angle of the signal 14c as received from the wafer 20. The scintillation counter 26 is connected to the sample stage 18 such that the wafer's position/angle can be controlled so that the angle of incidence (between X-ray 14b and the wafer) equals the angle of reflection (between X-ray signal 14c and the wafer 20). Once the angle of incidence of affected X-ray signal 14b is equal to the angle of reflection of 14c, it is possible for the scintillation counter 26 to orient the detector 22 and it's amplifier 24 such that it is normal to the point upon the wafer which is being tested for contamination.
The TXRF tool 10 requires approximately 1000 seconds in order to monitor a 4 mm.sup.2 location on a semiconductor wafer to determine whether or not a contaminant is present in this small 4 mm.sup.2 location. At less than 1000 seconds, an inadequate confidence level is obtained. Because a single 4 mm.sup.2 area on an 8 inch wafer is an insignificant surface area from which to determine whether or not contamination exists across an entire wafer, a number of regions need to be tested in a time serial manner by the device of FIG. 1. Typically, a minimum of five points on the backside of an 8-inch wafer would need to be tested in order to determine if excessive contamination is present on the wafer. Therefore, a total of 5000 seconds, or approximately 80 minutes, is needed in order to accurately determine whether or not a contamination potentially exists. The ramification of requiring this amount of time to determine whether or not contamination exists are several. One major ramification of the excessive detection time is that it is not feasible to test every wafer for contamination in an in-line process.
Therefore, statistical sampling needs to occur whereby one wafer every several hours would be pulled for testing. Worse yet, contamination may be checked only when a problem is detected with the tool. Both of these solutions may not detect hours of contamination thereby causing significant cross-contamination of fabrication tools and substantial yield reduction. Second, when a wafer is pulled for contamination check, other wafers are running through the tool in parallel to this testing. It would be possible for numerous wafers to be contaminated while the wafer is being checked due to the amount of time that has elapsed. For example, if a metal depositions/CMP processes takes 6 minutes per wafer, and the contamination detection takes 80 minutes per wafer; as many as 10 wafers could go through the metal deposition system and be adversely contaminated before the tested wafer could complete contamination analysis. Third, the limited area being checked on the backside of the area still results in the possibility that high levels of contamination or selectively placed contamination may exists on the wafer and go undetected. For example, if contamination were to exist on a portion of the edge of the wafer that was not chosen to be checked for contamination it would be possible for the wafer to be passed as a non-contaminated wafer, when in reality there was a significant amount of metal contamination which could cause "snowball" yield reduction if not corrected.
Therefore, it would be advantageous to develop a contamination detection system which: (1) is time-efficient in order to enable in-line contamination detection in a semiconductor manufacturing process; and (2) tests a significant portion of a wafer for contamination so that contamination detection is more comprehensive.